Can I use XPath functions with XPathNavigator?

Question

I've got an XPathNavigator at the root of a document. Several levels down, there's a group of numeric values that I want to sum. I could always loop through the nodes and add them myself, but since I knew the XPath spec included a sum function, I decided to try to use that. I'm running into an error.

System.Xml.XPath.XPathException - Expression must evaluate to a node-set.

Here's my code.

XPathDocument doc = new XPathDocument(new StringReader(myLiteralXML));
XPathNavigator nav = doc.CreateNavigator();
string myXPath = "sum(/root/level1/level2/elementsToAdd)";
XPathNavigator sumNode = nav.SelectSingleNode(myXPath);

I expected sumNode.Value to give me my sum (as a string). But instead I get the exception listed above when I try to populate sumNode.

I know my XPath is valid (or at least XMLSpy says it is). Can I not use the XPath functions on an XPathNavigator? Am I just doing it wrong?

(Also, just curious, am I wasting my time on a dead-end approach nobody's using, and risking not being able to figure out a year from now what this is doing?)

Answer 1

Try this instead:

XPathNavigator sumNode = nav.Evaluate(myXPath);

The XPathNavigator.Evaluate method is defined as:

Evaluates the specified XPath expression and returns the typed result.

as opposed to the XPathNavigator.SelectSingleNode method which is designed to only return nodes.